Transcriptional and Translational Control of Glucose‐Stimulated (Pro)insulin Biosynthesis

Abstract

Islets of Langerhans were isolated from the pancreata of fed or 48-h-fasted Wistar rats. The islets were incubated with either [3H]leucine of [3H]uridine. Inhibition of RNA synthesis by actinomycin D or by cx-amanitin for 4 h had no influence on the (pro)insulin biosynthesis of isolated islets of fed rats. The (pro)insulin biosynthesis was not inhibited after two days incubation of islets of fed rats with a-amanitin either. Incorporation of labelled uridine into total RNA for 3 h was stimulated by glucose in islets of fasted, but not of fed rats. Therefore, it was concluded that transcriptional control does not participate, even for longer periods than believed previously, in acute regulation of (pro)insulin biosynthesis of islets isolated from fed rats. Despite the strong and preferential stimulation of (pro)insulin biosynthesis of islets of fed rats by glucose the radioactivity of the [3H]uridine-labelled polysomes active in proinsulin synthesis remained unchanged.To interprete these experimental data we suggest that glucose triggers the transformation of a translationally inactive form of pre-proinsulin mRNA to a translationally active form.

The selective recruitment of mRNA to the ER and an increase in initiation are important for glucose-stimulated proinsulin synthesis in pancreatic β-cells

“…on the ER) demonstrates that the recruitment of the proinsulin mRNA to the ER plays a significant role in glucose-stimulated proinsulin synthesis. Contrary to previous reports [1,[3][4][5][6][7], we show that: (i) there is no detectable inert pool of 'free' proinsulin mRNA and (ii) glucose does not Abbreviations used: CPH, carboxypeptidase-H; DTT, dithiothreitol; ER, endoplasmic reticulum; ERK, extracellular-signal-regulated kinase; KRB, Krebs-Ringer bicarbonate buffer; MIN6, mouse insulinoma cell line 6; mRNP, messenger ribonucleoprotein; PC2, prohormone convertase 2; poly(A) + , polyadenylated; RNC, ribosome nascent chain; RRL, rabbit reticulocyte lysate; SRP, signal recognition particle; SR, SRP receptor; UTR, untranslated region. 1 To whom correspondence should be addressed (email tph4@le.ac.uk).…”

“…Moreover, ongoing translation is necessary for glucose-stimulated proinsulin secretion as inhibition of translation leads to defects in insulin secretion [30]. As glucose stimulates a greater fold increase in proinsulin synthesis compared with that of the majority of other proteins, including other secretory granule proteins, it is believed that glucose stimulates proinsulin synthesis by a specific mechanism [5]. However, the mechanism by which glucose stimulates secretory granule protein synthesis, and specifically proinsulin synthesis, is poorly understood.…”

“…These rapid increases in protein synthesis are mediated almost entirely at the translational level [3,5,6] and ensure that proteins released upon glucose-stimulated exocytosis are immediately replenished with newly synthesized proteins. Moreover, ongoing translation is necessary for glucose-stimulated proinsulin secretion as inhibition of translation leads to defects in insulin secretion [30].…”

“…To ensure the immediate replenishment of insulin, there is a rapid and specific increase in proinsulin synthesis (10-20-fold within 40 min) [2,3]. This acute increase in proinsulin synthesis is mediated via a posttranscriptional mechanism as it occurs both in the absence of any change in proinsulin mRNA abundance [3] and in the presence of transcription inhibitors [4][5][6]. However, the mechanism by which glucose rapidly stimulates proinsulin synthesis is poorly understood.…”

Pharmacological Agents That Directly Modulate Insulin Secretion

“…Translocation to the ribosomes and translation increase within minutes with glucose concentrations of Ͼ3.3 mM (Welsh et al, 1986). The rates of transcription and translation are not acutely coupled, demonstrated by the delay of several hours between inhibition of transcription and a dampening of translation (Jahr et al, 1980). Upon the translation of about 50 residues the nascent chain emerging from the ribosomal complex binds to the signal recognition sequence of an 11S ribonucleoprotein complex and the elongation is halted as the translation complex binds to the endoplasmic reticulum.…”